The present invention relates to a ceramic filter suitable for filtration of a molten metal, particularly, a molten ferrous metal, and to a method of filtering a molten metal using the ceramic filter.
A three-dimensional reticulated skeleton structure having interconnected pores, typically a ceramic foam produced from a synthetic resin foam has been known to be useful for filtration of a molten non-ferrous metal such as molten magnesium, aluminum or copper, or a molten ferrous metal such as molten ductile cast iron, gray cast iron or cast stainless steel. Such a filter has been generally formed into a rectangular or circular plate.
In the filtration of a molten metal, there is a difference between a molten ferrous metal and a molten non-ferrous metal in terms of the amount of the molten metal having passed through the filter for each filtration. To be more specific, the filtrated amount of a molten non-ferrous metal has been in a range from several ten tons to several hundreds tons; while the filtrated amount of a molten ferrous metal has been in a range from several ten kg to several tons. With respect to the usage of the filter, in general, the filter has been pre-heated for a molten non-ferrous metal; while it has not been pre-heated for a molten ferrous metal. Accordingly, as the filter used for a molten ferrous metal, a filter made from a ceramic material which is excellent in thermal shock resistance and has a structure in which the molten ferrous metal is smoothly passed therethrough is required. More specifically, there has been generally used a filter made from silicon carbide and having a structure being relatively small in the number of pores (i.e., relatively large in the size of pores) for reducing the resistance upon passing through the pores. Such a filter, however, has a problem in terms of strength if it is intended to make the thickness of the filter thinner for further reducing the resistance when the molten metal passed therethrough: and also has a problem in terms of corrosion resistance because the filtration of a molten ferrous metal in an amount of several tons takes a time of one minute or more. This corrosion occurs due to the reaction of silicon carbide with iron, and more specifically, pouring the molten metal for one minute or more may corrode the skeleton of the ceramic foam filter, thus the function of the filter is obstructed. In order to solve such an inconvenience, there is disclosed a technique to coat the surface of a skeleton of the ceramic foam filter with alumina being less reactive with iron, in Japanese Patent Laid-open No. Hei 5-51278; however, the coating of the skeleton with alumina is not yet sufficient to prevent corrosion of the skeleton.
With respect to the above filter for a molten ferrous metal, it has been proposed a method of increasing the total surface area of the filter for shortening the pouring time; however, such a method has a problem that since the filter is not pre-heated, the use of the filter having the increased surface area tends to reduce the temperature of the molten metal, leading to a failure in flow of the molten metal and also to breakage of the filter due to thermal shock. The problem has been solved by reducing the size of the filter and providing several gates to disperse thermal shock applied to each filter. This method, however, has presented another problem making difficult the design of a cavity of a casting mold, and in prolonging the filtrating time and making the corrosion resistance poor because of the presence of a plurality of the gates.
As described above, in filtration of a molten ferrous metal, particularly, a molten cast iron, since the molten metal must be poured in a casting mold for a short period of time, the related art filter having a rectangular or circular shape is poor in resistance against thermal shock and corrosion, particularly, when the molten metal in an amount of several tons or more is poured, resulting in the increased percent defective.